High lead and baria seal glass articles in suitable shapes such as, rods, billets, or preforms are used in forming seals either to bond parts of a structure together in a gas-tight manner such as, for example, in forming gas display panels or in sealing the outer surface of an article such as a magnetic recording head. The sealing materials have softening points which are low enough so that the temperature which is necessary to form the seal does not cause thermal damage to the parts which are being sealed or bonded. The glasses are a mixture of oxides whose composition is chosen so that they have the proper softening point and co-efficient of thermal expansion to be compatible with the surfaces being sealed. The tensile strength of these glasses is generally in the range of only about 10,000 - 15,000 psi. The sealing glass structures, therefore, must be handled with some care in order to avoid breakage. Also, it has been found that in sealing gas panel structures, in which sealing glass articles are placed between the two parts of the panel where they are subjected to heat and pressure in order to form a gas-tight bond between the parts of the panel, small pieces may separate from the surface of the seal glass preforms during the application of pressure. These pieces usually originate from bumps or seeds caused by the fact that the sealing glass may not be completely homogeneous. When these pieces are ejected from the sealing material into the space between the panel surfaces, they contaminate the structure so that a satisfactory gas panel will not result. There is a need, therefore, to find a way to increase the strength of the sealing glass articles. It has been found that the usual cleaning procedures with, for example, peroxides or etching with hydrofluoric acid, which have been used to improve the strength of some types of glasses, not only fail to improve the tensile strength characteristic of high lead or baria sealing glasses but actually have been found to cause it to deteriorate. Also, hydrofluoric acid treatment results in the formation of a white insoluble residue on the glass which flakes off such that it would be a possible source of contamination.
A process has now been found to substantially increase the tensile strength of sealing glasses.